Existing portable barcode readers are hand held and can be moved with respect to a target barcode, to image and decode the bar code. Target objects, e.g., a product package that includes a target barcode, are brought within a field-of-view (FV) of the barcode reader by aiming a visible aiming pattern to strike the package at a region of the barcode. In stationary bar code readers the situation is reversed, i.e. the product is moved through a stationary field of view. The barcode reader typically provides an audible and/or visual signal to indicate the target barcode has been successfully imaged and decoded.
Both stationary and portable imaging-based barcode readers include at an imaging system that includes at least one camera assembly for capturing image frames of a field of view of the camera assembly. A typical camera assembly includes a pixel or sensor array having photosensitive elements such as a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) device and an imaging lens assembly for projecting and focusing the field of view onto the sensor array. The camera assembly may be augmented with an illumination system having light emitting diodes (LEDs) or a cold cathode fluorescent lamp (CCFL) that directs illumination toward a target object. In certain cases, the target object may be a form or document, the extent of the form or document is sometimes referred to as a desired image acquisition area. The form or document may also include a target bar code within the confines of the form or document.
If the target object is within the field of view of the camera assembly, light reflected from the target object is focused through the imaging lens assembly such that focused light is concentrated onto the sensor array of photosensitive elements. The pixels of the sensor array are sequentially read out by image processing circuitry of the imaging system, generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. A digitized version of the captured image frame is stored in a memory, the digital values stored in memory for a captured image frame corresponding to light received during an exposure period by individual sensors or pixels of the sensor array.
The image processing system operates on the captured image frame to: 1) identify the imaged bar code within the captured image frame and attempt to decode the imaged bar code; and 2) identify the extent or bounds of the imaged form or document such that the portion of the captured image frame corresponding to the imaged form or document can be saved and/or subject to further processing, i.e. signature verification. Stated another way, in addition to identifying and decoding an imaged target bar code within a captured image frame, the image processing system additionally seeks to identify an area of the captured image frame (an array of pixels of the sensor array) that corresponds to or is congruent with the desired image acquisition area, that is, that corresponds to the form or document which was sought to be imaged.
One problem faced by designers of imaging-based bar code readers that are used for both bar code reading, that is, imaging and decoding a target bar code, and for image acquisition is that of accurately determining the extent of the imaged form or document. In conventional bar code readers that are used for image processing of a form wherein a bar code is located within the form, information is provided to the image processing system relating the size and location of the target bar code and the size and location of the form with respect to the target bar code. For example, information may be provided which relates the size and location of the form to a predetermined point of the target bar code, for example, the center of the target bar code. When the image processing system identifies the imaged target bar code within a captured image frame, the center of the imaged bar code can be identified and when combined with the previously supplied information regarding the size and location of the form with respect to the center of the target bar code, the image processing system can attempt to scale and extrapolate the extent or bounds of the imaged form within the captured image frame outwardly from the center of the imaged bar code.
The foregoing conventional method of scaling and extrapolating the extent or bounds of the imaged form is problematic. The size of the target bar code with respect to the size of the form may be greatly different, for example, the form may be 10 times (or more) the size of the target bar code within the form, thus, even small errors of a few pixels in determining the center of the imaged target bar code in a captured image frame will be magnified by a factor of 10 or more by the extrapolation process thereby causing unacceptably large errors in determining the extent or bounds of the imaged form. For example, if the corners of the imaged bar code are used as base points to predict or extrapolate where the corners of the imaged form will be, even small errors on the order of a few pixels with regard to the location of the corners of the imaged bar code will be greatly magnified when attempting to extrapolate out to the corners of the imaged form.
Thus, accurate identification of the extent or bounds of an imaged desired acquisition area, such as an imaged form, an imaged document, or an imaged label remains a challenge for designers of imaging-based bar code readers.